The present invention relates to the over-current protection of transistor devices and more particularly, to the over-current protection of power transistor devices utilizing a Darlington configuration.
Many electronic devices use transistors for regulating the electric current passing through a load. Circuits having output power transistors which are directly controlled by an exciter transistor are common and are known as Darlington circuits or Darlington configurations for a power circuit. The exciter transistor controls the output power of the circuit adequately under normal conditions, but in the event that such a device becomes overloaded, or if the load is short-circuited, the electric current through the transistor continues to increase in intensity. If the intensity increases beyond a certain point, the current can even destroy the output power transistor. Thus such a circuit requires some type of over-current protection for the power output transistor.
One method utilized in the prior art which provides the necessary over-current protection has been to include a sensing resistor connected in series to the Darlington configuration of transistors. As the current in the output circuit increases, a potential develops across the resistor. This potential may then be applied to a base-emitter junction of an auxiliary transistor, where the auxiliary transistor collector electrode is connected to the base electrode of an exciter transistor in the Darlington configuration. As the potential which has developed across the sensing resistor exceeds a predetermined threshold value, the auxiliary transistor becomes biased into conduction and acts as a clamp on the base drive. Thus the exciter transistor limits the base-emitter current of the output power transistor which in turn limits the current in the output circuit as a whole.
However, the prior art over-current protection circuit as described above tends to cause a significant increase in the active voltage losses of the circuit when it operates in the switching mode or when the transistors are saturated. These losses may be on the order of 20% or more and can significantly affect the efficient operation of the output power circuit.
Another example of an over-current protection circuit known in the prior art is shown in U.S. Pat. No. 3,845,405. The circuit described by this patent provides for over-current protection for a single transistor power device. Furthermore, the devices known to the prior art all lack the advantages provided by the present invention as is described below.